LENSES

Question 1

Principal focus (focal point):

Answer 1

the point on the principal axis where rays parallel to the
Principal axis (from a very distant object) will converge

Question 2

Focal length:

Answer 2

The distance between the centre of the lens and the principal focus (is negative for diverging lenses and positive for converging lenses)

Question 3

Image distance (v) :

Answer 3

The distance between the image and the centre of the lens (is negative for virtual images and positive for real images)

Question 4

Object distance (u):

Answer 4

the distance between the object and the centre of the lens

Question 5

Magnification:

Answer 5

v/u or image height/object height (no unit)

Question 6

Converging lens:

Answer 6

A lens that causes rays to converge on the principal axis – has a positive focal length

Question 7

Diverging lens:

Answer 7

A lens that causes rays to diverge after refraction – has a negative focal length

Question 8

Power=

Answer 8

1/ focal length . units = D (dioptres)

Question 9

What is unique about the Power of a diverging lens?

Answer 9

Power of a diverging lens is negative

Question 10

What is the power of multiple lenses equal to?

Answer 10

When multiple lenses are lined up on the principal axis, their powers add up (remember to subtract any diverging lenses)

Question 11

Virtual image

Answer 11

An image formed from the apparent divergence of rays from an object

Question 12

Real image

Answer 12

An image formed when real rays of light converge to form the image

Question 13

Explain how to estimate the focal length of a converging lens

Answer 13

Pass parallel rays of light from a distant object through the lens and move a screen
behind the lens until the image is focused. The distance between the lens and the
focused image is the focal length

Question 14

What equation links image distance,object distance and focal length?

Answer 14

1/u + 1/v = 1/f